1. Field of the Invention
This invention describes equipment for testing underground pressurized motor fuel piping, for leaks into surrounding soil. Environmental regulations promulgated by Federal and State law, require such leak detection equipment.
2. Description of Prior Art
Many commercial companies are now selling line leak detection equipment, or plan to in the immediate future. The equipment currently available, fall into design categories of mechanical and electronic-mechanical designs.
Using application programmed micro-computer technology, line leak detectors utilize flow and pressure decay technology to perform the EPA mandated leak test.
Whether pressure decay or flow is used, the application programmed micro-computer, must compensate for any temperature differential between the fuel and the pipe wall. All third party approved systems compensate for temperature differential for the 0.1 GPH and 0.2 GPH test, for the 3 GPH test, compensation is not required for the maximum temperature differential of .+-.25 degree F., as specified by the EPA.
In early winter or spring, in some locales, for a short time interval, temperature differentials may exceed the .+-. degree F. temperature differential as specified by the EPA. In that case all existing line leak detectors may false alarm. In the Fall false leak alarms will be reported, and in the Spring leaks will not be reported. This phenomenon occurs, due to warm underground fuel tanks, filling cold piping and with warm piping being filled from cold underground fuel tanks, thusly the first condition reports false alarms and the second condition will conceal a leak.
Pressure decay line leak detectors, relate a pressure drop over time, to a leak rate. For this method to be accurate, temperature differential between fluid and the piping wall must be compensated for. Additionally the effects of differing piping bulk modului must be considered, usually by increasing the duration of the pressure decay measurement.
Flow line leak detectors automatically null out the effects of the piping bulk modulus, accomplished by testing with a constant pressure source. However temperature differentials between the fuel and pipe wall must be compensated for. The test is conducted by diverting flow through a small orifice, and directly measuring any flow, which thereby represents a leak rate.
The strategies used to compensate for temperature induced errors, employ waiting, multiple tests or direct measurement of the pipe wall and fluid, for temperature, thusly determining if any differential exists.
Differing piping bulk modului can effect the accuracy of a pressure decay leak test method. Piping bulk modulus, can be determined by measuring the volume of fluid released from a predetermined pressure, and utilizing a predetermined pressure drop. The bulk modulus thusly obtained is used to determine the appropriate time of test. The test time calculated corresponds to the desired leak rate to be detected, for pipe with the bulk modulus determined. For continuous operating piping, the piping bulk modulus will be unchanged, over time and use.
For flow line leak detectors varying piping bulk moduli, effects on leak detection, is removed by the use of a constant pressure source, during the test interval.
Because gasoline has a thermal expansion rate 6 to 7 times that of water, thermal effects can be significant. Pressure decay and flow line leak detectors, will false alarm if temperature is not adequately compensated for.
Temperature differentials as small as 1 degree centigrade can cause pressure fluctuations, as great as 10 PSI, depending on the gasoline formulation.
Since gasoline is formulated to performance varying with weather and altitude, a preset formula cannot be used, to predict pressure change with temperature, increase or decrease for a typical gasoline grade.
To avoid inducing temperature increase from the fluid pulsing, induced by the turbine pump during pumping, care must be taken in the leak detector design to minimize that effect, (this applicable to precision testing in pressure decay devices). For the 3 GPH test, the pump is off, also the precision test, this approach is utilized by at least one line leak detector manufacturer. For the 0.1 or 0.2 GPH test, one line leak detector manufacturer, will test with the pump off and another with the pump on. Some manufacturers utilize the turbine pump check valve and pressure relief valve, to trap pressure, while others supply valving to trap pressure for the test.
Flow line leak detectors, are sensitive to sand in the fuel plugging the flow orifice. Most flow line leak detector manufacturers include in their designs, filters to protect the flow detecting orifice from clogging.
Flow sensing line leak detectors depend on the submersible turbine pump, to provide the constant pressure source required. The test is performed with the dispensing nozzle closed, the pump operating and the flow if any diverted through a flow restricting orifice. Any flow is determined to represent a leakage rate. This method removes the problem of varying piping bulk modului, but temperature differentials remain, to be compensated for. Detecting gross leaks (broken pipe) or leaks as small as 3 GPH is easily within the state-of-the-art for most line leak detection devices, when temperature extremes do not exceed the EPA .+-.25 degree F. differentials of the fuel to pipe wall temperature difference. However, the EPA requirement failed to recognize that in certain locales of the country, during early Fall or Spring, temperature differentials, can exceed twice the stated EPA criteria, for temperature extremes. During the conditions stated above, line leak detectors, now certified, will fail the EPA performance requirement for a reliability of 95% detection with 5% false alarms.
Detection of leaks in the range of 0.1, 0.2 and 3 GPH with reliability, for all conditions and locales, requires a knowledge of environmental conditions, beyond the parameters for test and certification, as promulgated by the EPA.
Additionally the ultimate performance of any line leak detection system cannot be adequately determined by any test protocol, without also factoring into the evaluation, the level of technician skill required to perform a correct installation. During third party tests, systems being evaluated are properly installed, and are used in situations within the systems performance limits. However in the real world, use of such equipment, if overly complicated in installation steps, will often lead to being improperly installed and thusly the systems performance will be degraded, to below required EPA standards.
Additionally for reliability levels to remain at the required level, the system must be easily retested for performance and easily readjusted if required. With this type of equipment, simplicity of installation is mandatory, if performance is to be maintained. With many line leak systems installed and third party certified, it is surprising that an optimum system that meets the requirements above stated, remains to be produced.
The following patents are applicable examples of the current state-of-the-art of pressure decay line leak detectors:
______________________________________ Reynolds; 3,935,567 January 27, 1976 Elmore III; 4,797,007 January 10, 1989 Michel Et Al; 4,835,717 May 30, 1989 Hill Et Al; 4,876,530 October 24, 1989 Slocum Et Al; 5,103,410 April 7, 1992 Hutchinson Et Al; 5,317,899 June 7, 1994 Filippi/Miller; 5,372,032 December 13, 1994 ______________________________________
The following are applicable examples of flow sensing line leak detectors:
______________________________________ Gerstenmaier Et Al; 4,131,216 December 26, 1978 Maresca, Jr. Et Al; 5,078,006 January 7, 1992 Maresca, Jr. Et Al; 5,090,234 February 25, 1992 Williams; 5,201,212 April 13, 1993 ______________________________________
Line leak detectors that utilize pressure decay for line leak detection are evaluated by the following statements;
Referring to Reynolds, the line leak detector therein described, fails to compensate for piping bulk modulus, temperature and differing pump off pressures. This device will not reliably find leaks below 3 GPH.
Referring to Elmore, III, the line leak detector therein described attempts to compensate for temperature, but fails to incorporate the micro-processor logic necessary for piping bulk modulus and variable pump off pressures.
Referring to Michel Et Al, the line leak detector therein described incorporates a micro-processor to interpret data from a temperature sensor, in order to compensate for fuel/pipe temperature differentials. The device fails to recognize that a single point temperature measurement, does not represent the temperature differentials along the entire length of piping. Additionally, this device fails to compensate for the piping bulk modulus and variable pump off pressures.
Referring to Hill Et Al, the line leak detector therein described incorporates various means to compensate for temperature, bulk modulus and variable pump off pressure, however, their performance is limited to a narrow range, that is insufficient for real world conditions. Temperature differentials between fuel and piping are detected by sequential testing, looking for a decrease in the time to a threshold pressure. When the decrease occurs, the effect is interpreted as due to temperature. Testing is continued until no difference is recorded in the time to the threshold pressure. If the time remains longer than 8 seconds no alarm is activated. This approach ignores the fact that sequential pump operation, itself introduces heat into the fluid, and thereby precludes an accurate measurement. Additionally, leaks below 3 GPH cannot be detected in the 8 second time and 5 PSI decay. Compensation for differing piping bulk modulus is determined by any time greater than two seconds to achieve 15 PSI, after pump shut down. This approach will only work over narrow limits, when the check/relief valves performance is tailored to a specific release rate. Compensation for variable pump off pressure, is attempted to be achieved by a spring loaded piston, delivering make-up fluid to the pressure trapped by the check/relief valve, when the pressure drop is below 12 PSI. This device is limited by its range of 4 to 11 PSI and its fluid volume of 5 cubic inches. The attempt is to persuade the pump off pressure to always be 10 PSI. Considering spring force degradation over time and the make-up volume limit, not to mention the spring force degradation of the pressure relief valve, it certainly is an impossible task.
Referring to Slocum Et Al, the line leak detector therein described incorporates many of the features of Michel Et Al, with some improvements. The primary improvement being an anti-thwart switch, that prevents continuously resetting the leak detector, by laying a brick on the reset switch. This patent does not address or describe any means, whereby any temperature differential, along the entire length of piping, are sensed. A single point or even a multiple fluid temperature sensing points, may miss a section of piping wherein significant temperature gradients may exist. In that situation a probe may indicate no temperature, wherein in fact there is a temperature gradient, further down the pipe which may create significant pressure differentials. A practical example of such a condition would be piping from a dispenser under a canopy. The canopy shades the piping thereunder, but does not shade the piping as it terminates to the underground tanks. In that case a temperature sensor under the canopy, would not sense the same temperature as the fuel in the piping that is not shaded, but in fact is warming up. This condition poses a significant restraint to finding a precision leak, with the leak detector herein described. The patent additionally fails to perform the precision leak test on any system that does not have a pump off pressure between 8 to 14 PSI. The Slocum leak detector tests from 4 to 3 PSI to determine a precision leak. For a pump with an off pressure equal to the pumping pressure, the device would not perform the precision test. For a pump with an off pressure equal to the pumping pressure, the device would not perform the precision test. For a pump with an off pressure condition that must be compensated for, even at a 3 GPH test. This patent mentions the piping bulk modulus, (air) and describes different test times, but fails to relate the test time to a bulk modulus value. For fuel piping in continuous operation, the piping bulk modulus exhibits a constant value. However, the value must be determined, and the leak detector must have a capability, to adjust its performance in accordance with the pipings particular bulk modulus.
Referring to Hutchinson Et Al, the line leak detector therein described specifically excludes the 3 GPH leak detection and claims a 0.1 GPH leak detection; on the surface this seems to be a contradiction of values. However, upon further examination, it becomes apparent that the patent cannot detect comparatively large leak rates, because of its method of operation.
This patent teaches leak detection by pressure decay monitoring with the pump on and pressure trapped between pump and dispenser. When the dispenser valve is closed, initiation of the submersible turbine pump creates a pressure spike, due to the hammer effect of the STP start-up. This initial pressure surge is higher than the normal pumping pressure of the STP.
A combination of check and pressure relief valve traps this dynamic pressure surge. Assuming the normal pumping pressure is X, then the dynamic pressure is X+X.sub.1. X.sub.1 is 10 to 20% of X. The check valve traps pressure since X+X.sub.1 pressure is greater than X pressure.
The pressure relief valve is set to relieve at 2/3 X pressure against it. Whereas, the pressure trapped side of the pressure relief valve has X+X.sub.1 pressure opposing the closed position. Since X+2/3X is always much greater than X+X.sub.1, the opposing forces keep the pressure relief valve closed. Thereby the trapped pressure is monitored for a pressure decay value representing a 0.1 GPH leak.
However, for a leak rate of 3 GPH, which is 30 times greater than a 0.1 GPH leak, the process will not work! With such a comparatively large leak the dynamic pressure spike is never trapped because it is constantly being relieved by the 3 GPH leak in the pipe.
The result is the check valve allows flow into the leaking pipe 3 GPH or more, maintaining the pipe pressure, but not allowing the pressure decay required, to signal a large leak (3 GPH).
Referring to Filippi/Miller; the line leak detector therein described compensates for temperature and pipe bulk modulus, but fails to compensate for temperature extremes greater than .+-.25 degree F. and fails to keep installation simple.
Line leak detectors that utilize flow, for piping leak determination, exhibit different performance characteristics than pressure decay devices. However, these devices are as sensitive to the primary obstacle to accurate line leak detection, that is temperature differential, as are pressure decay devices.
Referring to Gerstenmaier Et Al, the line leak detector therein described utilizes a flow detector in a bypass piping arrangement, thereby determining a leak. The test is commenced after 30 minutes of no product pumped through the piping under test. This design is the precursor of later designs using a micro-computer for control. The fundamental fault with this design, relates to temperature stabilization of the fluid under test. The 30 minute wait without pumping product, is insufficient by several order of magnitudes. Additionally flow is determined at a constant pressure, thereby requiring the pump to be on, during the duration of the test. Pulsation of the fluid under test defeats the desired normalized fluid temperature, by introducing heat into the fluid under test.
Referring to Maresca, Jr. Et Al; patents dated Jan. 7 and Feb. 25, 1992, the line leak detector therein described, utilizes a modified flow concept. The intent is to eliminate heat induction into the test fluid, during the test cycle. For flow detection methods to work, a constant pressure is required during the flow test. The line leak detector therein described, uses a pressure chamber with an air head, to provide a relatively constant pressure during the flow test. Sets of 3 tests are performed. Flow is determined by the change of height of the test fluid, in the pressure chamber. The first and third tests are flow tested at the selected test pressure, their results are averaged. The second test is performed at zero pressure. It is assumed at zero pressure, no leakage will occur from a potential hole in the piping. The flow if any is then attributed to thermal expansion or contraction of the fluid. Flow again is measured by the change of column height in the test chamber. The average flow from test 1 and 3, is corrected by adding or subtracting test 2, depending on contraction or expansion of the test fluid, from thermal effects. An assortment of valves, pressure chamber, controllers, flow switches, piping, cabling are utilized to obtain the desired performance. The subsequent patent improves with a positive displacement pump and other methods. In addition to the overall complication of this approach, the assumption that fluid will not be lost from a leaking pipe at zero pressure, ignores the inherent pressure head, from the location of the pressure chamber.
Referring to Williams; the line leak detector therein described, does not fit into the category of a permanently installed leak detector. The patent teaches a technology for pressurized line test that is performed yearly, for detecting 0.1 GPH leak. This technology because of setup time and complication, would be impossible to use for the required 3 GPH test on a continuous basis.